It was more than just a warning. The Unites States had a major presence in the Pacific. The Japanese wanted to gain control of areas in the west and southwest Pacific and east and southest Asia that would supply them with the natural resources Japan lacks to build and maintain a strong industrial and military power - coal, iron ore, oil, nickel, manganese, rubber etc. The US military, especially the Navy, had to be removed for threir plans for conquest to succeed. They attacked Pearl Harbor, the Philippines, Wake Island and attempted to take Midway Island. They also attacked and conquered Malaysia, including British held Singapore, planned to take New Guinea and attack Australia. They had to drive the British out of the Pacific/Asia area too. Yes the US cut off shipments of scrap metal and oil to Japan, but I personally don't think that was that big of an issue. The Japanese had all ready (1937) invaded Manchuria in China which was a major producer of iron ore and coal. What percentage of a nations metal needs can be met by scrap iron? I don't know but not very high I would think. (It is true that Roosevelt was a strong Anglophile and was itching to get in the war to support the British. But whether he ended oil and scrap metal shipments to Japan to prod them to attack us, I dont think anyone knows that for sure.